In motor vehicles, radar sensors are used in conjunction with driver assistance systems, for example, distance warning and control systems, and for position finding of objects, in particular other vehicles, in the surroundings of the host vehicle. For example, long-range radar (LRR) sensors which operate at a frequency of approximately 77 GHz as well as short-range radar (SRR) sensors which operate at a frequency of 24 GHz are common. When the radar sensor is installed in the front of the vehicle and is used to measure the distance from preceding vehicles, the purpose of the beam-shaping device is to bundle the emitted and/or received radar radiation, at least for a forwardly directed radar lobe, in such a-way that the maximum intensity and sensitivity are achieved in the angular range in which preceding vehicles are normally present, whereas objects located farther from the roadway produce little or no radar echo. For a radar sensor which operates by angular resolution, multiple radar lobes are generated which fan out in the azimuth, thus allowing conclusions to be drawn concerning the azimuth angle of the located object based on the amplitude and phase relationships between the signals received from the various lobes.
In one common design of radar sensors for motor vehicles, the beam-shaping device is formed by a spherical lens or, in a more general sense, a lens in the shape of a rotational solid which is made of a plastic which has a high index of refraction for the particular frequency of the radar radiation and which bundles the radiation in the manner of a focusing lens. Thus, essentially the same directional characteristic is achieved in the azimuth (in the horizontal direction) and in the elevation (in the vertical direction). For radar sensors, which operate by angular resolution and which have multiple adjacent antenna elements situated in the focal plane of the lens, interference between these antenna elements may result in a certain modification of the directional characteristic in the azimuth.
However, for long-range radar sensors it is often desirable to bundle the radiation in the elevation more strongly than in the azimuth, so that on the one hand a sufficiently wide field of vision is achieved in the azimuth, but on the other hand, as the result of stronger bundling in the elevation unnecessary energy losses are avoided while at the same time it is possible to better suppress interfering signals resulting from reflection from the roadway surface or the like. One way to achieve such an anisotropic directional characteristic, i.e., one that is different in the elevation compared to the azimuth, is to use complicated lens systems having multiple lenses. However, in radar sensors for motor vehicles this is not feasible for cost reasons and because of the large space requirements for the lens systems.
On the other hand, compact radar sensors having a planar design are known in which the beam is shaped not by use of optical lenses, but, rather, by a suitable selection of the geometry, configuration, and phase angle of multiple antenna elements, for example by use of so-called group antennas, phased arrays, or Rotman lenses (WO 2006/029926). Although the configuration of the antenna elements may be selected in such a way that different directional characteristics are achieved in the azimuth and in the elevation, the numerous antenna elements require a large amount of space on a relatively expensive high-frequency substrate, resulting in relatively high costs for radar sensors of this type, in particular when a high-quality beam-shaping device is to be implemented which allows a symmetrical directional characteristic and good suppression of side lobes.